ChaCha20 CTR Encryption

[btoms20]

Added a method to interact with BoringSSL's CCryptoBoringSSL_CRYPTO_chacha_20()

Checklist

• [x] I've run tests to see all new and existing tests pass
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [ ] I've updated the documentation if necessary

Motivation:

Having the ability to explicitly set the Counter in the ChaCha20 Cipher allows us to generate 'Header Protection Masks' as described in RFC 9001 - Using TLS to Secure QUIC and issue #168

Modifications:

Added a single encryption method under the Insecure.ChaCha20CTR enum that lets the user of this library perform single block encryptions with an explicitly defined Counter and Nonce / IV.

The ChaCha20CTR enum described above is defined in the _CryptoExtras product.

Result:

After including both Crypto and _CryptoExtras in your project you'll have access to the ChaCha20CTR enum and the associated encryption method that enables one off encryption operations.

import Crypto
import _CryptoExtras

Insecure.ChaCha20CTR.encrypt(
   message: DataProtocol,
   using: SymmetricKey,
   counter: Insecure.ChaCha20CTR.Counter,
   nonce: Insecure.ChaCha20CTR.Nonce
) throws -> Data { ... }

See the included ChaCha20CTRTests.swift file for usage examples.

[Lukasa]

@swift-server-bot test this please

[Lukasa]

@swift-server-bot test this please

[Lukasa]

Excellent, one quick fixup: can you run scripts/update_cmakelists.sh and commit the changes?

[Lukasa]

@swift-server-bot test this please

[btoms20]

Awesome! Thanks for guiding me through this PR, I really appreciate all the comments / help. Sorry for taking up so much of your time! 😅